The term "in situ" metal matrix composites or MMC is used herein to distinguish such aluminum alloys from aluminum alloy composites reinforced by the physical addition of carbide or other hard particles to aluminum, such as silicon carbide, titanium carbide, alumina, TiB.sub.2, and the like. The MMC alloys are generally referred to as dispersion strengthened alloys.
"In situ" or "natural" MMC are produced by taking advantage of the chemical and physical metallurgical characteristics of special alloys. This type of MMC is formed during solidification of an alloy with the proper elemental content, e.g., a eutectic or near-eutectic composition, the subject of this development. At the beginning of processing, the alloy is entirely molten and is prepared using conventional fusion metallurgy. During solidification of the alloy, the reinforcing phase forms naturally in the structure. It is only after the melt solidifies that the discrete phases of the composite can be identified in the microstructure.
A eutectic is an invariant point on the equilibrium phase diagram of an alloy containing two or more components. In a binary componant system, liquid and two solid phases can only coexist at the eutectic temperature and the eutectic composition. Each component can contain more than one element and eutectic mixtures containing more than two phases are possible.
The term eutectic used herein is meant to include near-eutectic compositions as well as all eutectic compositions. Generally speaking, these eutectic compositions will contain some amount of primary phase.
Eutectic alloys generally possess various microstructures and properties depending on how the alloy is solidified and worked. Special solidification processing techniques, such as casting under a unidirectional thermal gradient, can be employed to align a reinforcing phase in a eutectic; these directionally-solidified or aligned eutectics are highly anisotropic and have attractive properties in the as-cast state. In the present invention, no special solidification processing is employed or required; conventionally cast ingots of eutectic alloys are mechanically worked after casting to produce composite properties.
Simple static or direct-chill casting followed by rolling or extrusion of eutectic alloys is a process which is relatively inexpensive and easily adaptable to current aluminum production capability. These alloys are suited for applications where enhanced stiffness, strength, low density, and good ductility are required. The static casting and working produce sheet or bar with moderate levels of anisotropic (directional) behavior which is useful in these applications. In addition, statically-cast and worked eutectics are superplastic, for example, when hot formed at low or medium strain rates, and thus extensive forming deformation of large parts at low loads is possible without premature fracture.
There are several other advantages to producing a composite by the eutectic route which are given as follows:
The eutectic melt contains all the ingredients, e.g., no reinforcing solids must be added to the melt.
Depending on the specific phases, the interface between the matrix and reinforcing phase in an eutectic MMC is entirely coherent, or at least semicoherent and free of environmental contaminants or (interfacial) phases which can have undesirable effects on mechanical properties.
The size and spacing of the reinforcing phase in many eutectic composites is often more refined as compared to that in a conventional MMC which can have a desirable effect on strength, stiffness, and ductility.
Depending on the specific phases, the eutectic MMC can be rolled, forged, or extruded more easily than a conventional MMC containing comparatively brittle reinforcements. Also, depending on the specific phase, the reinforcement itself in the eutectic MMC will have some ductility in combination with a high modulus.
Depending on the specific matrix phase and chemistry, post solidification solution and aging heat treatment can be employed to alter the structure and properties of the eutectic composite.
So long as temperatures are not exceeded that would alter the morphology of the reinforcing phase or the matrix state of precipitation, the eutectic composites can be utilized at elevated temperatures.
One method of producing dispersion strengthened aluminum alloys is disclosed in U.S. Pat. No. 3,989,548 which issued on Nov. 2, 1976. According to the patent, the method disclosed is directed to the production of such alloys by forming a casting of a specified composition in which brittle rod-like intermetallic phases are present, following which the casting is mechanically worked to break up the rod-like phases and form separate particles which are dispersed through the mass. The patent states that when intermetallic particles of a size within the range of about 0.1 to 2 microns form from a 5 to 20% by volume of an aluminum alloy, the worked alloy possesses interesting mechanical properties. When the volume fraction of the phases falls below 5%, the mechanical properties are inferior, and that when they exceed 20 volume %, the ductility and toughness decline.
A similar disclosure appears in a sister application of the above-identified patent which issued as U.S. Pat. No. 4,126,486. This patent is directed to a method and product, the composition consisting essentially of 7-10% Si, up to 1% Cu, up to 1% Mg, up to 1% Mn and the balance aluminum, with up to a total of 1% residuals. The method comprises producing an aluminum-silicon alloy by continuous casting in the form of a thin slab at a growth rate of about 25 cm/min. so as to solidify silicon in the form of elongated rods of about 0.05 to 0.5 microns uniformly dispersed throughout the thickness of the slab. The slab is subjected to 60% reduction to fragment the silicon rods into finely divided separate particles and later cold-rolled to at least a final 10% reduction to final sheet form, following which the cold-rolled sheet is annealed at a temperature in the range of 250.degree. C. to 400.degree. C.
A disadvantage of the foregoing methods is the initial requirement of massively mechanically working the alloy to a reduction of at least 60% in order to fragment the hard silicon phase into fine particles.
We have discovered a method for producing wrought aluminum eutectic composites without first massively mechanically working the alloy to break up elongated intermetallic particles or phases to produce a fine dispersion thereof.